1. Field of this Invention
This invention lies in the field of additives to increase the fusion temperature of slag derived from solid fuel.
2. Description of the Prior Art
The use of additives for the purpose of gaining some performance advantage in fuel combustion has been given much attention. The additives generally may be classified as (1) preflame additives (for improved storage and handling of fuel), (2) combustion additives (for improved combustion efficiency and reduction of pollutants and particulates), and (3) post-flame functioning additives (for particulate collection, fireside deposit control, and cold side corrosion reduction). In the present invention, the emphasis is on class (3) fuel additives, especially those which control the deposits of slag including in commercial-sized furnaces using coal and other solid fuels.
Slagging and fouling (sometimes termed fireside deposits) and corrosion problems are generally considered to be caused by the sulfur, alkali-metals, and chlorine in solid fuels such as coal. Fundamental research is still needed to understand the mechanisms involved.
Among known class (3) additives are magnesium based additives, such as magnesium oxide, metallic magnesium particles, and the like, which can be added to slag and slag precursers by spraying or injecting into a combustion chamber. Reduction in deposits in both oil and coal fired boilers has been reported.
Silica based additives reportedly reduce ash bonding and such additives are used mostly with residual fuel oil.
Antimony trioxide is reportedly an effective fuel additive in reducing deposits and corrosion, but antimony is a toxic substance.
Copper oxychloride is reported to have been successfully used in coal fired boilers to produce light, friable, easily removed deposits (in place of previously obtained heavy, glassy slag deposits).
Calcium based montmorillonite clay reportedly has been used as a combustion improver and slag preventative.
During combustion in a boiler furnace, the reactions which occur between the fuel impurities lead to the formation of deposits which adhere to boiler surfaces. Such deposits upset the normal operating conditions and produce problems by causing: (a) obstruction to gas flow, (b) interference with heat transfer (resulting from the insulative nature of such deposit), (c) damage to water tubes on the ash slopes particularly with bulky such deposits), (d) corrosive conditions producing serious loss of metal tubes. These deposit and associated corrosions force nonscheduled outages for cleaning (deslagging) and/or for replacing the failed tubes which can be costly.
Even though a lot of research and experimental work has been carried out on the problem of so-called external fouling in boilers, there still remains a large number of unsolved problems. The prior art method for reducing boiler fouling involving the use of fuel additives has been used with varying degrees of success. In oil fired boilers, additives help mainly to prevent corrosion on the cooler parts of the system, while in coal or bark fired boilers the emphasis has been on additives which reduce external fouling.
New and improved class (3) fuel and slag treatment additives are needed to minimize the slagging and the deposit producing (fouling) potentials of the combustion products of coal and other combustible organic solid fuels.